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A TEXT-BOOK OF BOTANY. 



of the anther and the discharge of the pollen, and this layer is 

 called the endothecium (Fig. 79, end). There is still a third or 

 external layer of cells, which constitutes the exothecium (Fig. 79, 

 ex). These four sporangial regions may remain more or less 

 distinct and separate at maturity, or the two on either side may 



Fig. 79. Development of pollen sacs (microsporangia) in several of the Angiosperms: 

 A, showing beginning of archesporium (a), an outer sterile layer (b), position of connective 

 (con); B, later stage showing development of fibrovascular tissue (gf); C, longitudinal 

 section of archesporium; D, E, F, successive later stages showing in addition pollen mother 

 cells (sm) and tapetum layer (t). G, H, diagrammatic sections of mature pollen sacs show- 

 ing pollen mother cells (pm), tapetum (t), endothecium (end), exothecium (ex), and in H 

 longitudinal dehiscence with formation of what appears to be a unilocular pollen sac on 

 either side of the connective. — A-F, after Warming; G-H, after Baillon and Luerssen. 



coalesce. ' This latter usually occurs at maturity, when dehiscence 

 takes place, forming apparently a single pollen sac on either side 

 of the connective or axis (Fig. 79, H). 



The Microspores (pollen grains) are developed somewhat 

 differently in Monocotyledons and Dicotyledons. In most mono- 



